In the step of polishing synthetic quartz glass substrates for use as photomasks, liquid crystal substrates or the like, polishing compositions containing cerium oxide (ceria) or colloidal silica are often used.
Since synthetic quartz glass substrates for use as photomasks, liquid crystal substrates or the like are required to have a high flatness, high smoothness and low defectiveness, several steps including lapping and polishing steps are carried out for surface adjustment until the substrates are ready for use as the product.
The lapping step is to remove the residual strain by slicing of an ingot into substrates. The polishing step is to polish substrates to mirror finish to increase their surface flatness and tailor their topography. The final polishing step uses a polishing composition of colloidal silica with a smaller particle size to produce substrates having a flat and smooth surface and devoid of microscopic defects.
In the step of polishing substrates to mirror finish, a ceria polishing composition is commonly used and effective. Under the current situation that has faced a steep rise in rare earth prices, engineers make extensive efforts to develop replacement polishing compositions or recovery techniques for the purpose of reducing the amount of ceria used.
For example, Patent Document 1 discloses a polishing composition obtained by mixing ceria with zirconium oxide (zirconia) to form complex oxide particles. Glass substrates are polished using this polishing composition. Since Patent Document 1 does not completely free the composition of ceria, the polishing composition is not regarded as a complete ceria replacement polishing composition. Mixing of zirconia invites a change in zeta potential on surfaces of ceria particles and zirconia particles, leaving concerns that the dispersibility of the polishing composition is exacerbated and more residues are formed on the surface of substrates as polished.
Patent Document 2 describes that using colloidal silica particles of non-spherical shape and varying aspect ratio, semiconductor wafers are polished at a higher polishing rate than the use of conventional colloidal silica. In Patent Document 2, as SiO2 originating from glass substrates binds with colloidal silica particles, colloidal silica particles gradually approach spherical shape, inviting a drop of polishing rate. In addition, since colloidal silica particles are of irregular shape or varying aspect ratio, they are effective for suppressing scratches to the substrate surface, but tend to form microscopic defects which can be killer defects in photomask substrates.